In particular, the present invention relates to an array antenna for radar sensors used in motor vehicles in connection with driver assistance systems, for example in order to locate vehicles driving in front in an automatic distance regulating system (ACC: Adaptive Cruise Control). These radar sensors typically operate with a frequency of 24 GHz or 77 GHz.
In these applications, planar antennas that use microstrip technology have the advantage that they can be produced at relatively low cost and enable a flat design, and that no transitions are required between different line systems or line types. Like the other circuit components, the antenna elements can be formed simply by corresponding microstrip conductors on a circuit board.
In an array antenna, or antenna array, the power that is to be radiated is fed into the various antenna elements in such a way that a desired directional characteristic results through superposition and interference of the radiation emitted by the various antenna elements. For this purpose, the power levels and phases of the microwaves introduced into the individual antenna elements must be tuned in a suitable manner. As a rule, a directional characteristic is desired having a narrow main lobe and largely suppressed secondary lobes. Since, however, only limited space is available for the radar sensor and thus also for the antennas, a complete suppression of the secondary lobes can for the most part not be achieved.
In a conventional array antenna of the type named above, the feed lines to the individual antenna elements branch off from the supply line, and at the branching points so-called transformers are provided in the supply line that, through a suitable transformation of line impedances, bring about the desired distribution of power to the antenna elements. In the case of antennas in microstrip technology, the transformers are typically formed by line segments having differing widths and having a length that corresponds to one-fourth of the wavelength of the microwaves on the line.
However, the transformers in the supply line are disturbance points that cause undesired emissions and reflections. This not only limits the possibility for suppressing secondary lobes, but also makes more difficult the suppression of cross-polarizations, which is often desirable for example in order to prevent disturbance by other systems. Normally, the radiation emitted and received by the radar sensor has a particular polarization, for example a linear polarization in a particular direction. Cross-polarization is understood as a radiation component having a polarization orthogonal thereto.
Due to limitations in the forming of the branches, it is for the most part also not possible to distribute all the power supplied via a supply line completely to the antenna elements, and to radiate it completely via the antenna elements. For the most part, excess power remains on the supply line, and must either be radiated or destroyed at the end of this supply line. The radiation of the undesired power, however, again causes more pronounced secondary lobes. Destruction of the excess power using special absorbers has the disadvantage that the absorbers create additional costs and require additional space. Moreover, this solution has the disadvantage that higher overall power losses occur.